Lightened the background color.

The original jellybeans.vim color scheme does not use a black
background, but a dark grey.  Looking at the code, the RGB value is
151515, so I've changed ujelly's background color to 141414.

References:
https://github.com/nanotech/jellybeans.vim/blob/master/colors/jellybeans.vim#L301
http://nanotech.nanotechcorp.net/downloads/jellybeans-preview.png